阳木虎年(male Wood-Tiger)−239 or −620 or −1392— to —阴木兔年(female Wood-Rabbit)−238 or −619 or −1391

Year 366 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Mamercinus and Lateranus (or, less frequently, year 388 Ab urbe condita). The denomination 366 BC for this year has been used since the early medieval period, when the Anno Dominicalendar era became the prevalent method in Europe for naming years.

Theban leader, Epaminondas, returns to the Peloponnesus for a third time, seeking to secure the allegiance of the states of Achaea. Although no army dares to challenge him in the field, the democratic governments he establishes there are short-lived, as pro-Spartan aristocrats soon return to the cities, reestablish the oligarchies, and bind their cities ever more closely to Sparta.

Thebes makes peace with Sparta and then turns its attention on Athens, which is trying to revive its maritime empire and is interfering in Macedonian dynastic quarrels.

The experiment by Dion (brother-in-law of Dionysius I) and Plato to educate the new ruler of Syracuse, Dionysius II, in the practical application of Plato's philosophical principles fails and Dion and Plato are banished from Syracuse.

The use of military tribunes with consular power is abandoned permanently and the dual consulship is restored. A new magistracy is established, which is called the praetorship. Its holder, the praetor, is elected annually by the Assembly and takes charge of civil matters, thus relieving the consuls of this responsibility. The praetor is regarded as a junior colleague of the consuls. Nevertheless, the praetor can command an army, convene a Senate or an assembly, as well as exercise the consular functions.

Two additional aediles, called curule ("higher") aediles, are created in the Roman hierarchy. These are at first patricians; but those of the next year are plebeians and so on year by year alternately. They are elected in the assembly of the tribes, with the consul presiding.